1. Field of the Invention
The present invention relates to a non-contact three-dimensional shape measuring method for measuring a three-dimensional shape of an object to be measured by projecting predetermined measurement light onto the object to be measured, and receiving light reflected thereon, as well as a non-contact three-dimensional shape measuring apparatus, and a focus adjusting method suitable for the method and the apparatus.
2. Description of the Related Art
Three-dimensional data concerning an object to be measured can be obtained by a non-contact three-dimensional digitizer adopting e.g. a light projection method, which is also called as a light cutting method. The three-dimensional digitizer is adapted to acquire three-dimensional data concerning an object to be measured, based on imaging data obtained by projecting slit pattern light onto the object to be measured and receiving light reflected thereon (see e.g. Japanese Unexamined Patent Publication No. 2005-321278).
The three-dimensional digitizer is provided with an auto-focus (AF) function of a light receiving optical system with respect to the object to be measured. Passive AF of obtaining a focus position based on an imaging contrast of a two-dimensional image which has been obtained by imaging an object to be measured, and an active AF of obtaining a focus position by projecting laser light onto an object to be measured, receiving the laser light reflected thereon, and metering the light by the triangulation principle are widely used as the AF function. Generally, the three-dimensional digitizer has a certain depthwise dimension for measurement (hereinafter, called as “measurement depth”) including a front position and a rear position with respect to a focus position, as measurement dimensions. The position of the measurement depth with respect to the object to be measured is adjusted depending on a focus operation using the AF function.
Japanese Unexamined Patent Publication No. Hei 5-107463, Japanese Examined Patent Publication No. Hei 8-16608, and Japanese Unexamined Patent Publication No. 2000-283739 disclose examples of the methods for acquiring distance information concerning an object to be measured. According to these methods, the distance to an object to be measured is measured by projecting pattern light onto the object to be measured, and distance information is acquired by using different pattern images. It is possible to adjust the position of the measurement depth with respect to the object to be measured, using the aforementioned distance measuring method based on pattern light projection.
The passive AF fails to provide sufficient contrast in the case where the ambient light around an object to be measured is dark, or in the case where the object to be measured is a flat member such as an outer body of an automobile. In the above cases, a focus position error may be unduly increased. On the other hand, the active AF control is advantageous in accurately obtaining the focus position. However, the position of the measurement depth may not be properly adjusted depending on a point on the object to be measured having a three-dimensional shape, as a metering point. Specifically, in the case where the object to be measured has a relatively large projected surface or recessed surface, it is desirable to adjust the position of the measurement depth in such a manner that both of the projected surface and the recessed surface are included in the measurement depth. If, however, the projected surface is selected as the metering point, the recessed surface may be out of the measurement depth, which resultantly fails to measure the shape of the recessed surface. In this case, a measurement of the recessed surface is required after a position adjustment is performed in such a manner that the recessed surface is included in the measurement depth, which may increase the number of times of the measurement.
According to the distance measuring method based on pattern light projection, in view of a necessity that different pattern images are used to obtain distance information, it is required to perform an imaging operation multiple times, while changing the pattern light, and it takes a long time to perform an imaging processing. Accordingly, a time may be needed to adjust the position of the measurement depth, even if the distance information concerning the object to be measured is obtained.